ES_SVVent User Manual, Rev. 1

1. Introduction

2. Methods and Formulae

 

2.1 Compressible Steam Flow Analysis

 

2.2 Type of Vent Stack

 

2.3 Relieving Steam Condition

 

2.4 Back Pressure

 

 

2.4.1 Superimposed back pressure

 

 

2.4.2 Built-up Back Pressure

 

2.5 Design Pressure

 

 

2.5.1 Design Pressure of Discharge Elbow

 

 

2.5.2 Design Pressure of Open Discharge Type Vent Stack

 

 

2.5.3 Design Pressure of Closed Discharge Type Vent Stack

 

2.6 Flow between Discharge Elbow and Vent Stack

 

2.7 Reaction Force

 

 2.8 Dynamic Load Factor

3. Major Screens

 

3.1 Input Screen

 

3.2 Menu  

 

3.3 Iso-metric Screen of Discharge Elbow

 

3.4 Iso-metric Screen of Vent Stack

 

3.5 Text Output Screen

4. Test Run Results


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1. Introduction (TOC)

ES_SVVent is the program to analyze safety valve vent stack..    ES_SVVent has functions to analyze both open and closed discharge type vent stacks.   Discharge elbow of safety valve is also analyzed as a separate piping in case of open discharge type, while the discharge elbow of closed discharge type is analyzed as an integral piping of vent stack.   Other auxiliary calculations are also performed simultaneously, such as reaction forces, preliminary reaction forces of closed type during initial blow. etc.

2. Methods and Formulae (TOC)

2.1 Compressible Steam Flow Analysis

Compressible steam flow analysis of discharge elbow and vent stack is performed by the method developed by ENGSoft Inc. and presented in Compressible Flow Analysis of Steam web page.

Since the specific volume of compressible fluid is considerably big, the pressure variation due to elevation change is negligible so that all pipes with same internal diameter can be calculated as one pipe regardless of their elevations.   Using this concept, the program investigates the pipe data, sums the resistance coefficient(K) of the pipes with same ID and calculate the pipes as one pipe.   When pipe ID changes by increaser or reducer, the piping system is divided at the increaser or reducer and analyzed separately. 

Analysis is progressed from the piping exit to the inlet, inversely to the flow.   This is because the choked flow at the piping exit should be determined first and then the upstream condition is calculated progressively based on the downstream condition selected by comparison with critical pressure calculated.    For example, iIf there is no increaser or reducer in the piping system, the entire piping system is analyzed as a pipe, while analyzed in two pipes if there is one increaser or reducer in the piping system.

The program calculates also the safety valve nozzle area for reference and the safety valve nozzle critical pressure for checking the vent stack inlet pressure.   The safety valve nozzle is analyzed according to the Clause 5.1 Nozzle of Compressible Flow Analysis of Steam web page.

 

2.2 Type of Vent Stack (TOC)

There are two common types of vent stack.   The one is open discharge type and the other is closed discharge type.   Sometimes, the open discharge type is called as umbrella type and the closed discharge type as direct-connected type.

The discharge elbow and vent stack is physically separated in the open discharge type, while two pipes are physically connected by a increaser or reducer in closed discharge type.

For details about vent stack, please refer to Ref. No. 1.

 

2.3 Relieving Steam Condition (TOC)

In the program, the relieving steam condition of safety valve is selected as below.

Relieving pressure (P0)

Relieving pressure is selected as being same with the set pressure of the safety valve.

As described in Ref. No. 3, the maximum pressure of actual relieving steam pressure is higher than the valve set pressure.   However, it is not considered necessary to use the higher pressure because the product of pressure and specific volume which governs the compressible flow analysis, will vary insignificantly when the enthalpy remains unchanged.

Relieving temperature (T0)

Relieving temperature is selected as being same with the normal operating temperature upstream of the safety valve.

Relieving flow (W)

As a default, 117 % of the code stamped capacity of safety valve is selected as the relieving flow.

According to ASME Section VIII, Division 1, UG-131, the code stamped capacity of a safety valve is 90% of the average capacity of a set of three valves tested, and the tested capacity of a set of three valves shall fall within the range of plus or minus 5% of the average capacity.    Therefore, the maximum flow expected of a safety valve can be calculated as 100% / 0.9 / 0.95 = 117%.

The default percentage can be adjusted in the [Set]-[Calc] menu.

 

2.4 Back Pressure (TOC)

Back pressure is the static pressure existing at the outlet of a safety valve due to pressure in the discharge system.    The back pressure is classified in two categories, one as built-up back pressure and the other as superimposed back pressure.    The back pressure has an effect on the capacity and set pressure of a safety valve.   The built-up back pressure sometimes called as dynamic back pressure, while the superimposed as static back pressure.

The built-up back pressure is the pressure existing at the outlet of a safety valve caused and built up by the relieving flow friction resistance, and the superimposed back pressure is the static pressure existing at the outlet of a safety valve when the valve starts to relieve.   The built-up back pressure has an effect on the safety valve capacity, while the superimposed back pressure has an effect on the safety valve set pressure.

When the safety valve relieves to the atmosphere, the superimposed back pressure of the safety valve is the atmospheric pressure.    However, if several safety valves relieve to the atmosphere through a common discharge piping system, the superimposed back pressure of a safety valve may be higher than the atmospheric pressure when the other valves relieves at the same time.

Please note that the back pressure of a safety valve is the pressure at the exit flange of the safety valve, not the exit flange of discharge elbow.

 

2.4.1 Superimposed Back Pressure (TOC)

The set pressure of a safety valve is expressed as gauge pressure because the superimposed back pressure is normally atmospheric pressure.   However, when the superimposed back pressure is higher than atmospheric pressure, the set pressure of a safety valve is changed by an amount of the superimposed back pressure over atmospheric pressure.   That is, the actual set pressure or opening pressure of a safety valve is equal to the spring setting(cold differential test pressure) plus the superimposed back pressure.   For example, if the spring set pressure of a safety valve is 1000 psig and the superimposed back pressure is 30 psig, then the actual set pressure of the safety valve is 1030 psig.

Eventually, the superimposed back pressure effects on the safety valve set pressure.

When the superimposed back pressure is variable, a bellows type safety valve is normally used in order to avoid the effect of the superimposed back pressure to the valve set pressure.   Bellows type safety valve has a disk stem covered by a bellows, which prevents the force by the superimposed back pressure from exerting on the back side of the disk.

If a silencer is installed at the exit of vent stack, the silencer can be neglected in the compressible flow analysis of vent stack, because the velocity inside of silencer is in the range of normal velocity of steam pipe and the pressure drop by the silencer is negligible.   At the exit plane of vent stack pipe connected to the silencer, choking occurs.   Therefore, the compressible flow in vent stack can be analyzed with the assumption that the superimposed back pressure of silencer is imposed on the exit of vent stack.

If a silencer is installed in the middle of vent stack, the vent stack should be analyzed in two parts, separating the vent stack at the location of silencer.  First, analyze the part of vent stack downstream of silencer and then analyze the upstream part using the inlet pressure of downstream part as the superimposed back pressure of the upstream part.   Since the inlet pressure of downstream part is not known at the beginning, the analysis should be performed by try-and-error method.    Start from the middle pressure between the relieving pressure of safety valve and the superimposed back pressure of the vent stack.  The relieving temperature can be gotten from steam table using the pressure selected and total enthalpy of relieving steam.   Please note that the total enthalpy value is kept constant along the length of vent stack.

 

2.4.2 Built-up Back Pressure (TOC)

Overpressure or accumulation exists in safety valves.

The relieving flow of a safety valve is proportional to the clearance area between disk and nozzle, and the clearance area is made by the relieving pressure force overcoming the spring force.   When a safety valve begins to open, the spring force increases by compression, and the relieving pressure should also increase to overcome the increased spring pressure.    For this reason, a safety valve has an overpressure called an accumulation to reach full opening.   This over-pressure varies from 3% for valves on fired vessels to 10% for valves on unfired systems.

Built-up back pressure which occurs after a safety valve is open, has an effect on the force balance of disk and subsequently the disk lift and relieving flow.   Manufacturers recommend to use bellows type safety valves if the built-up back pressure is higher than the safety valve accumulation.

Meanwhile, please note than safety valve nozzle is selected under the condition of choked flow through the nozzle.   That is, the capacity of a safety valve is selected for the back pressure being less than critical pressure.   Therefore, in any case the built-up back pressure of a safety valve should be less than the critical pressure of the safety valve nozzle.

 

2.5 Design Pressure (TOC)

2.5.1 Design Pressure of Discharge Elbow

Ref. No. 1 recommends to use the upstream pressure of discharge elbow as the design pressure of the elbow.   However, ENGSoft Inc. recommends to use 110% of the upstream pressure of discharge elbow as the design pressure.

 

2.5.2 Design Pressure of Open Discharge Type Vent Stack

Ref. No. 1 recommends to use the upstream pressure of open discharge type vent stack as the design pressure of the vent stack.   However, ENGSoft Inc. recommends to use 110% of the upstream pressure of vent stack as the design pressure.

 

2.5.3 Design Pressure of Closed Discharge Type Vent Stack

Ref. No. 1 recommends to use at least 2 times of the upstream pressure of closed discharge type vent stack as the design pressure of the vent stack for a shock wave at the vent stack during initial stage of relieving.

ENGSoft recommends the same, too.

 

2.6 Flow between Discharge Elbow and Vent Stack (TOC)

The flow between discharge elbow and vent stack is analyzed by using the method described in Clause 5.3 Compressible Flow through Increaser and Reducer in Compressible Flow Analysis of Steam web page.

In case of open discharge type, the flow between discharge elbow and vent stack is analyzed as an increaser or reducer flow having the cross-sectional area suddenly enlarged.

Additionally in case of open discharge type, a calculation to check steam blow-back at the vent stack entrance is required as described in Ref. No. 1.

 

2.7 Reaction Force  (TOC)

Reaction force caused by steam velocity at each open end is calculated as described in Ref. No. 1.

Additionally reaction force by steam hammering during initial steam relieving stage at closed discharge vent stack  is calculated as below.

According to Ref. No. 1 recommendation, the design pressure of closed discharge type vent stack be at least two times of the steady state operating pressure at vent stack inlet for transient flow when the safety valve is initially opened.   Actually the force caused by the transient flow should be analyzed by a separate steam hammering analysis program.   However, at initial engineering stage it is useful to calculate preliminary hammering forces for piping design.

That kind of preliminary steam hammering forces may be calculated using the recommended design pressure given at Ref. No. 1 as below.

The maximum reaction force by hammering is same with the momentum change of fluid column with the length reached by pressure wave for unit time, and the maximum pressure in the conduit is caused by the momentum change.   Therefore, if the maximum pressure is known, then the maximum reaction forces can be calculated.   The design pressure recommended in Ref. No. 1 may be used as the maximum pressure in preliminary transient reaction force calculation.

The maximum reaction force during transient flow happens when the straight pipe length can catch full length of pressure wave.   If straight pipe length is shorter than full pressure wave length, the reaction force is reduced proportional to the pipe length.    If straight pipe length is longer than full pressure wave length, the reaction force is same with that of full pressure wave length pipe.

According to the method described above, the pipe reaction forces of close discharge type vent stack during initial safety valve opening may be calculated by the following equation.

Fi = PRRdgn * (Pi_avg - Pexh) * A * L / Vs / t_vo

where,

Fi

: Reaction force, kgf

 

PRRdgn

: Design pressure multiplier to steady state operating pressure

 

Pi_avg

: Steady state operating pressure of pipe, kgf/m2

(Average pressure of pipe inlet and outlet may be used.)

 

Pexh

: Vent stack pressure before safety valve opening., kgf/m2

(Normally atmospheric pressure)

 

A

: Pipe cross-sectional area, m2

 

L

: Pipe straight length, m

(If L > (Vs * t_vo), then L = Vs * t_vo)

 

Vs

: Sonic velocity of the fluid contained in vent stack before safety valve opening, m/sec

(In case of air, 340 m/sec)

 

t_vo

: Safety valve opening time, sec (Normally 0.04 - 0.05 sec.)

 

 2.8 Dynamic Load Factor   (TOC)

In a piping system acted upon by time varying loads, the internal forces and moments are generally greater than those produced under steady-state application of the load, and this amplification is normally expressed as the dynamic load factor, DLF.   According to Ref. No. 1, for structures having essentially one degree-of-freedom and a single load application, the DLF value will range between one and two depending on the time history of the applied load and the natural frequency.   The discharge elbow of open discharge type vent stack has one degree-of-freedom and therefore its DLF will be the value between one and two.

Meanwhile, for closed discharge vent stack the analysis is not simple and requires a time history analysis of the piping system.

DLF may be expressed by the following equation.

DLF = Maximum Amplified Reaction Force / Steady-state Reaction Force

 

3. Major Screens   (TOC)

3.1 Input Screen

 

The input screen above is for open discharge type and the followings are required for user to input.

-

Pipe iso-metric files for discharge elbow and vent stack

-

Safety valve set pressure

-

Operating steam temperature

-

Safety valve rating

-

Superimposed back pressure

In case of closed discharge vent stack, input of discharge elbow iso-metric file is not required because discharge elbow if exist may be input as a part of vent stack with a increaser or reducer.   When user select [Closed Discharge Type] at menu [Set]-[Calc], the iso-metric file input window is automatically disappeared.   

A check box is prepared for input of dry saturated steam at valve set pressure.

For open discharge the superimposed back pressure is fixed as standard atmospheric pressure user can not change, while the back pressure can be altered by user in case of closed discharge.

The input of discharge elbow and vent stack pipe iso-metric information should be performed using the sub-program of ES_PipeIso, of which user manual you may find in ES_PipeIso User Manual web page.   [Open] command button is used for opening a existing *.pip file, while [Edit] command button is used to open [ES_PipeIso] window for editing a already-opened *.pip file.    A existing *.pip file may be open in [ES_PipeIso}window using its own file open menu.

When *.pip file is open by [Open] command button, the units of the *.pip file keep their original values, while the calculation in the mother program is performed in the mother program's units.    When *.pip file is open in [ES_PipeIso] window, the units of *.pip file are converted into the mother program's units, and then if user saves the *.pip file the units of *.pip file are saved as converted.

Please note that the *.pip file has to be saved before exiting [ES_PipeIso] window in order to apply the changes made in the window to the calculation in  mother program.

3.2 Menu     (TOC)

[File] menu has [New], [Open], [Save], [Save As] and [Exit] items with four file items lately used.

[Run] menu has only [Start] item which uses function key [F5] as a short key.

[Set] menu has [Title], [Unit], [Calc], [Text Output] and [Graph Output] items.

In [Set]-[Title] item, two titles may be input as [Title 1] and [Title 2], which also are shown in Text Output and Graph Output.

In [Set]-[Unit] item, the unit of calculation is set.

[Set]-[Calc] item includes,

Vent stack type

Open

Discharge elbow and vent stack are physically separated, and so the pipe iso-metric files should be input separately, too.    Program automatically checks steam blow-back(leak to ambient) between discharge elbow and vent stack.

Open is default setting.

 

Closed

Discharge elbow and vent stack are physically connected, and so only vent stack pipe iso-metric file is required for input.    The increaser or reducer between discharge elbow and vent stack is generated automatically in [ES_PipeIso] window by differing vent stack pipe nominal diameter from that of discharge elbow.   For closed discharge vent stack, the preliminary reaction forces by initial transient flow are calculated by program.

 

 

 

Safety valve discharge coefficient

Except high pressure dry saturated steam

ES_SVVent program calculates safety valve nozzle area, in which the method of Nozzle Clause of Compressible Flow Analysis of Steam web page is used.   Discharge coefficient input by user is used to get the actual safety valve nozzle area by dividing the theoretical value by discharge coefficient.

As explained in Clause 2 of ES_StmNzl and ES_StmPipe Program Test Results web page, the high pressure dry saturated steam above 1500 psig deviates maximum from theoretical values.   In this concern, a separate input is prepared for high pressure dry saturated steam.

Default values are 0.98 and 0.94 respectively.

Meanwhile, set pressure plus accumulation is used for nozzle area calculation according to reference document, while set pressure is used for vent stack analysis.

 

High pressure dry saturated steam

 

 

 

Over-pressure or Accumulation

Percentage over set pressure (%)

Default value is 3%.

 

 

 

Vent stack design mass flow rate

Percentage to safety valve stamped rating (%)

Default value is 117%.

 

 

 

Design pressure multiplier(% to calculated inlet pressure)

 

Open discharge type

Default values are 110% and 200% respectively.

Closed discharge type.

 

 

 

Transient analysis data for closed discharge type

Safety valve opening time, sec

Default value is 0.05 sec.

 

Sonic velocity ratio to air for other fluid in vent stack

Default value is 100%, which is for air of 340 m/sec.

 

 

 

Dynamic load factor(DLF)

 

Reaction force output of the program is the value with dynamic load factor multiplied.   Default value is 2.

Meanwhile, the initial reaction force output of closed discharge type due to transient condition is not multiplied by DLF because the reaction forces are not steady-state forces.

[Set]-[Text Output] has two check box sub-menu of [show details of permanent pipe] and [show details of temporary pipe].

[Set]-[Graph Output] item shows a form to set the parameters of graph output.

3.3 Iso-metric Screen of Discharge Elbow   (TOC)

When calculation result does not exist, the pipe iso-metric screen of discharge elbow shows only iso-metric drawing, while shows inlet and outlet pressure also when calculation result exists.

For closed discharge type vent stack, this screen is inactivated because the pipe iso-metric data of discharge elbow is not input by user separately..

Although the pipe length of discharge elbow of open discharge type vent stack is relatively short, the program also analyze the pipe using the method developed by ENGSoft Inc.

The nozzle area of a safety valve is selected by manufacturer in the condition that the back pressure of the valve is not higher than critical pressure in any case.   Therefore, the discharge elbow and vent stack of a safety valve should be designed for the valve back pressure not to exceed the critical pressure of the valve nozzle.    ES_SVVent program calculates the critical pressure of the safety valve using the method developed by ENGSoft Inc. and then provides user alarm if the inlet pressure of discharge elbow is higher than the critical pressure calculated.    User alarm is given in this screen as well as in the text output screen described below.

As described in Clause 2.4.2 safety valve manufacturers recommend to use bellows type safety valves if the built-up back pressure is higher than the safety valve accumulation.   ES_SVVent  program warns user to consult with manufacturers for using bellow type safety valve when the calculated inlet pressure of discharge elbow is higher than the accumulation or over-pressure of the safety valve.   User warning is given in this screen as well as in the text output screen described below.

The iso-metric window can be printed.

 

3.4 Iso-metric Screen of Vent Stack   (TOC)

When calculation result does not exist, the pipe iso-metric screen of vent stack shows only iso-metric drawing, while shows inlet and outlet pressure also when calculation result exists.

For open discharge type vent stack, the program checks the momentum forces at vent stack inlet in order to assure that no steam is blown back to ambient.    If the momentum force check resulted in steam blow-back, then the program provides user alarm.    User alarm is given in this screen as well as in the text output screen described below.

Since the iso-metric screen of discharge elbow is inactivated for closed discharge type vent stack, the warning messages for higher back pressure of closed discharge type are given in this screen.

The iso-metric window can be printed.

 

3.5 Text Output Screen   (TOC)

[Text Output Screen] shows the calculation result details and can be printed.

The followings are shown.

1.

Safety valve design inputs

2.

Other design inputs

3.

Relieving steam conditions

4.

Safety valve nozzle

5.

Discharge elbow

6.

Vent stack

 

4. Test Run Results   (TOC)

Test run results of ES_SVVent for the Clause 6.0 Sample Calculation of Ref. No. 1 are as below.

Description

Ref. No. 1

ES_SVVent

Discharge elbow

Inlet pressure(P1a), psia

194

182

 

Exit pressure(P1), psia

118

121

 

Exit velocity(V1), ft/sec

2116

2044

 

Exit reaction force(F1), lbf

12801

12683

 

 

 

 

Vent stack

Inlet pressure(P2), psia

77.4

77

 

Exit pressure(P3), psia

51.4

51

 

Inlet velocity(V2), ft/sec

1507

1490

 

Exit velocity(V3), ft/sec

2116

2098

 

Inlet reaction forces(F2), lbf

12645

12537

 

Exit reaction forces(F3), lbf

11861

11755

 

 

 

 

Steam blow-back

Momentum force, lbf

2201

2001

 

Pressure force, lbf

2030

1855

You can find out that the calculation results of ES_SVVent are well in line with those of Ref. No.1.   Ref. No. 1 uses different coefficients depending on steam conditions when calculating critical pressure and sonic velocities, which resulted in more accurate calculation than other methods in which experimental equations with rather simple coefficients are used.

 

References :  (TOC)

1. ASME B31.1-1992, Appendix II Nonmandatory Rules for The Design of Safety Valve Installations

2. Fluid Flow, A First Course in Fluid Mechanics, Second Edition by Rolf H. Sabersky, Allan J. Acosta and Edward G. Hauptmann, The Macmillan Company, New York, 1971

3. Analysis of Power Plant Safety and Relief Valve Vent Stacks by G.S. Liao, Bechtel Power Corp., Transactions of the ASME, 1974

4. Crosby Pressure Relief Valves Engineering Handbook, Crosby Gage & Valve Company, March 1986

5. Steam Hammer in Power Plant Piping Evaluation and Restraint Design Optimization by M.Z. Lee and E.C. Goodling of Gilbert/Commonwealth Inc., June 1982


Copyright (c) 2000 - 2001 ENGSoft Inc., Seoul, Korea, All right reserved.